It’s Alive! Biological Soil Crusts in the Sonoran and Chihuahuan Deserts


In the sublime expanses of the Sonoran Desert you might not expect that some of the
most interesting life around can be found at the dirt right for your feet. But if you can tear
your eyes away from the sunset and fix them on the ground for a few minutes you may discover a diverse community of
tiny organisms, all working together to perform vital
ecosystem functions. Biological soil crusts form a living
ground cover that is the foundation of desert plant life. Biological soil crusts, or BSCs, are made up of cyanobacteria, which sometimes look like areas of “dirty dirt” on the ground. BSCs also include scaly lichens and
furry mosses, as well as green algae, microfungi, and
bacteria. The visual appearance of BSCs varies by region. In the Sonoran and Chihuahuan deserts, BSCs tend to be flatter and less
charismatic than the black, knobby crust more characteristic of
the Colorado Plateau, or the more lush communities found in
the northwestern US. Like many other life forms, desert BSCs can often be found growing under a shrub or bush that provides shelter from
the sun and wind. The primary function of BSCs is to hold
the soil surface together. When wet, cyanobacteria move through the soil and bind rock or soil particles together, forming
a web of fibers. Mosses and lichens have small anchoring
structures that hold the soil in place. All of these factors help stabilize the soil, increasing its resistance to wind and
water erosion. BSCs don’t even have to be alive to continue their work. Layers of abandoned sheaths, built up
over long periods, can still be found clinging
tenaciously to soil particles, providing stability in sandy soils up to
10 centimeters deep. Other BSCs that appear to be dried
out seem to come alive when doused with water, like this moss. Dry and grey when found, a
sprinkling of water causes it to become metabolically active again. In addition to holding the soil
in place, cyanobacteria are also nitrogen-fixers,
meaning that they are able to convert atmospheric nitrogen to a form plants can use. This is
especially important in desert ecosystems, where nitrogen levels are
low and often limiting to plant productivity. Soil crusts also intercept and store water, nutrients, and organic matter that might
otherwise be unavailable to plants. Unfortunately, many human
activities can be harmful to soil crusts. Trampling and crushing caused by footprints or machinery are extremely harmful, especially when the crusts are dry and
brittle. Tracks in continuous strips, like those caused by vehicles or bicycles, form areas that are highly vulnerable to
wind and water erosion. Rainfall then carries away this material,
causing channelization– especially on slopes. Impacted areas may
never fully recover. Although a thin veneer of cyanobacteria may
return in a few years, lichens and mosses may take up to fifty
years to regrow. The best way to avoid damaging soil
crusts is simply to stay away from them by always driving and riding on
designated roads and trails and steering clear of roadside vegetation. When hiking, always walk on marked trails or on other durable surfaces, such as rock or in sandy washes. Enjoy these tiny communities with care,
and don’t bust the crust!

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